The present application relates to gas turbine engines, and more particularly, to bound assemblies disposed along the gas flow path of gas turbine engines.
Within the core of the gas turbine engine, working gases flow along a gas flow path, which in various sections of the engine can be defined by an inner case and an outer case. The inner case is disposed radially inward of the outer case with respect to the centerline of the gas turbine engine. Both cases are commonly comprised of a plurality of ring shaped structures that are assembled and connected axially to one another to form the housing/casing that defines the gas flow path. A plurality of airfoils comprising static vanes and rotor blades are disposed within the gas flow path along the compressor and turbine stages to extract mechanical work from the working gases. With high bypass turbofan engines, bound assemblies such as static ring/strut/ring assemblies are disposed in the gas flow path at various stages including in or adjacent the fan section, compressor section, turbine section, exhaust section, and diffuser. Ring/strut/ring assemblies can be thought of as bound assemblies because the strut is connected to both the inner case and the outer case. Bound assemblies are commonly used to provide structural support to one or both of the cases or to bearings which support the shafts that rotate within the engine. Bound assemblies such as struts are also used in some applications for aerodynamic and/or noise reduction purposes within the gas flow path.
Gas turbine engines are continually undergoing changes with the goals of improving performance, decreasing size and weight for a given thrust rating, while reducing cost and enhancing durability and repairability. To improve performance, it is typical to increase the operation temperature of the engine, since increased temperatures generally will translate into improved engine performance. As a result of the increased temperatures, the components disposed in and adjacent to the gas flow path are subjected to increased temperature gradients.
Increased temperature gradients, and temperature gradients in general, pose a particular problem for bound assemblies because the gradients typically result in the struts being heated to a greater degree than the inner case and outer case. This differential heating creates a thermal growth differential between the struts and inner case and the outer case, which results in the struts expanding to a greater degree than the cases. In particular, the thermal growth differential makes the strut attempt to expand radially outward with the expansion of the inner case. The amount of this expansion differs from the amount of expansion of the outer case, which expands to a lesser degree. However, barring a catastrophic failure, the strut remains connected to both the inner case and outer case during thermal induced expansion, with the result being a thermal fight or “punch load” that typically causes high strains in or near the curved fillets that connect the cases with the struts. These high strains limit the number of thermal cycles the bound structure can be exposed to before experiencing cracks in or near the fillets. The cracks limit the useful service life of the bound structure.